The objectives of this randomized, double-blind chemoprevention trial are to determine whether chemopreventive treatments with soy protein reduce or delay further rise in PSA in men that present with a PSA failure as the first sign of disease recurrence following radical prostatectomy for prostate cancer. A second objective is to determine whether chemopreventive treatment with soy protein delays prostate cancer recurrence following radical prostatectomy, as indicated by PSA failure in men who are at high risk for recurrrence because their cancers have unfavorable characteristics. The ability of soy protein to reduce the frequency of recurrence and the quantity of circulating prostate cancer cells detected by RT-PCR will be determined.